1. Field of the Invention
The present invention relates in general to a dielectric filter for the microwave spectrum of frequency and a method of adjusting the frequency characteristic of the dielectric filter. More particularly, the present invention is concerned with a small-sized dielectric filter constructed for excellent filtering properties, and a method by which the frequency characteristic of such dielectric filter can be easily adjusted.
2. Discussion of the Prior Art
In a microwave telecommunication system of modern vintage such as a portable or automobile telephone system, various filters using dielectric ceramics are used for minimizing the transmission loss. A known dielectric filter has a plurality of coaxial type resonators connected to each other. Each resonator is a dielectric block which has a central through-hole whose cylindrical surface is metallized to provide a central conductor serving as a resonating element. However, the central through-holes of the resonators have been a limiting factor to an effort to reduce the thickness and size of this type of dielectric filter. Further, this dielectric filter has a relatively large number of parts, and accordingly requires a cumbersome or complex fabrication process.
On the other hand, a three-layered or so-called tri-plate type dielectric filter as disclosed in laid-open Publication No. 59-51606 of unexamined Japanese Patent Application, for example, is free from such drawbacks. Namely, it is recognized in the art that the tri-plate type dielectric filter can be comparatively easily fabricated, with a considerably reduced thickness. An example of the dielectric filter of the tri-plate construction is illustrated in FIGS. 12 and 13. This dielectric filter, which is indicated generally at 2 in FIG. 12, has a dielectric substrate 6 in which there is embedded a patterned array of an input and an output electrode 3 and a plurality of stripline resonator electrodes 4 (three electrodes 4 in this specific example). The outer surfaces of the dielectric substrate 6 are coated with a ground conductor 8 (respective conductive films 8), except certain areas on a pair of opposed side surfaces, on which an input and an output contact 10 are formed, respectively. Thus, the dielectric filter 2 is fabricated to be considerably compact and thin.
In the known tri-plate type dielectric filter 2 shown in FIG. 13, the resonator electrodes 4 are formed so as to provide a comb-shaped or interdigital structure, and the desired filtering properties are obtained by adjusting the spacing between the adjacent resonator electrodes 4. That is, the dielectric filter 2 does not have a circuit for electrically connecting the resonator electrodes 4. However, the applicants recognized a need for providing such an electrically connecting circuit so as to provide capacitors between the adjacent electrodes 4, in order to meet recent stringent requirements for improved properties of the dielectric filter for the microwave frequencies, which cannot be dealt with by the mere provision of a simple comb-shaped or interdigital structure of the resonator electrodes.
Conventionally, the final fine adjustment to obtain the desired frequency characteristic of the dielectric filter 2 is accomplished by trimming a portion of the ground conductor 8 which corresponds to the resonator electrodes 4, or by trimming the short-circuited ends of the electrodes 4 that are electrically connected to the conductor 8. However, the positions of the electrodes 4 embedded in the dielectric substrate 6 cannot be accurately detected, and it is difficult to achieve the desired frequency characteristic of the filter by trimming.